1. Field of the Invention
The present invention relates to a blanket substrate and a blanket, having the substrate, for offset printing, etc.
2. Description of the Background
A blanket, formed as a laminate comprising 3 or 4 fabric layers and a rubber layer, and having a smooth rubber layer for contact by ink, has been widely used in the past in offset printing. In order to prepare blankets having good printing characteristics, high-quality blanket substrates must be used. Accordingly, blanket substrates are required to have high-level properties of (1) good dimensional stability with little xe2x80x9celongationxe2x80x9d, (2) good adhesiveness to rubber layers, and (3) uniform thickness.
Blanket substrates with poor dimensional stability, under the load of machine driving, will elongate. This elongation lowers printing accuracy. In order to ensure the quality of the printed images desired, the xe2x80x9celongatedxe2x80x9d portion of the substrate must be wound up, which, however, interferes with efficient printing operation. On the other hand, the substrates which have low adhesiveness to rubber layers and the substrates whose thickness is not uniform give uneven prints, which results in unsatisfactory printing accuracy.
In order to prepare substrates which have uniform thickness and good adhesiveness to other layers, high-quality yarn of Egyptian cotton has been widely used, which, however, is problematic, because the use of this cotton for production of the substrates requires particular treatment with wet heat in order to enhance the dimensional stability of the substrates. In addition, even after the treatment, the dimensional stability of the substrates is lowered when they are again wetted. As a result, in repeated offset printing, blankets having a substrate are elongated whenever they are pressed against rolls and wetted. If the degree of xe2x80x9celongationxe2x80x9d is too large, the blankets must be re-tightened, for which the printing operation must be stopped. In addition, the xe2x80x9celongationxe2x80x9d changes the thickness of the blankets, which change results in non-uniformity of blanket thickness. The maintenance of the blankets requires much labor. Blankets, if not maintained well, will have poor printing characteristics.
Given this situation, polyvinyl alcohol (PVA) fibers having good dimensional stability and having high affinity for rubber have been proposed as blanket substrates (see JP 47-32908 and JP 62-282986). However, since ordinary PVA based fibers have a cocoon-like cross section, blanket substrates comprising them are problematic, because they often do not have uniform thickness, and, in addition, when used for a long period of time, they often lose xe2x80x9cresistance of cycle compressionxe2x80x9d and their capabilities become unsatisfactory.
Specifically, JP 47-32908 proposes using spun yarn prepared from PVA based fibers in order to prevent the elongation of blankets. However, the fibers constituting the spun yarn exhibit xe2x80x9cinterfiber slipsxe2x80x9d when a mechanical load is applied thereto. After all, therefore, even though high-strength fibers having a high modulus of elasticity are used for substrates, xe2x80x9celongationxe2x80x9d of the substrates comprising them is inevitable.
On the other hand, JP 62-282986 proposes the use of high strength, low-elongation PVA filament yarn for blanket substrates. In the proposed method disclosed, the substrates produced will not elongate very much, but their adhesiveness to other layers is low because the surface of the filament yarn from which the substrate is prepared has no nap. Therefore, these substrates do not ensure satisfactory printing accuracy.
In order to solve the problems noted above, the use of core yarn prepared by applying short fibers onto the surf ace of synthetic filaments or long staple fibers having a length of from 10-30 cm has been proposed for blanket substrates (see JP 63-249696 and 6-297877). These disclosures state that the core of the core yarn prevents the xe2x80x9celongationxe2x80x9d of the blanket substrates comprising the core yarn, and that the short fibers on the surface of the core improve the adhesiveness of the substrates to other layers. In practice, however, it is difficult to produce homogeneous, high-quality core yarn. Therefore, the substrates comprising core yarn are still problematic, because their thicknesses will not be uniform. In addition, since the short fibers which exist on the surface of the core will have xe2x80x9cinterfiber slipsxe2x80x9d, the dimensional stability of the substrates cannot be improved to a satisfactory degree. A need, therefore, continues to exist for an improved substrate for blankets used in printing operations.
Accordingly, one object of the present invention is to provide a blanket substrate which has the properties of good dimensional stability, uniform thickness, resistance to cyclic compression and good adhesiveness to rubber.
Another object of the invention is to provide a blanket which is comprised of the substrate.
Briefly, these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a substrate for a blanket which comprises a spun yarn of polyvinyl alcohol based fibers, wherein the fibers have primary ridged streaks which are formed on their surf ace in the direction of the fiber axis with finer secondary ridged steaks formed in the primary ridged streaks, and have a cross-section circularity of at least 80%.